Motor neuron diseases (MNDs) are neurodegenerative disorders that cause muscle weakness and often respiratory failure and death. Rapid progress in the molecular genetics of MNDs has revealed at least 22 distinct genes that are expressed in all cells and yet result exclusively in motor neuron (MN) loss when mutated. The small heat shock protein B1 (HSPB1, formerly HSP27) is mutated in patients with hereditary motor neuropathy (HMN). HSPB1 is unique among MND-causing genes in that overexpression of the wild type HSPB1 is known to be neuroprotective in MNs whereas mutations are toxic to MNs. The primary goal of this proposal is to determine the molecular function of HSPB1 that is relevant to motor neuron survival. To do this, we have developed a mouse model of motor neuropathy expresing the most common HSPB1 mutation (R136W) in neurons. We find that HSPB1 mutant mice display a phenotype of mild weakness that mimics HMN. We propose to develop a second line of mice expressing mutant HSPB1 in all cells so that we may distinguish between MN and non-MN contributions to MN injury. The role of non-neuronal cells in the progression of MND is emerging as an important concept and genes expressed by microglia in particular may be important targets in reducing MN loss in MNDs. We hypothesize that animals expressing HSPB1(R136W) in all cells will have a phenotype that is more severe than animals expressing HSPB1(R136W) exclusively in neurons. HSPB1 is required for a specific mRNA decay pathway caled AU-rich element (ARE)-dependent mRNA decay. AU-rich element mRNA decay is a critical mechanism in all cells to control the expression of a select group of mRNAs. Our preliminary data demonstrate that HSPB1(R136W) is defective in this RNA decay pathway, which raises the possibility that ARE-containing mRNAs (normally degraded via this pathway) may play a role in MN pathology. Many of these genes encode proteins such as interferons and inflammatory cytokines which have protective functions during injury and infections, but can be damaging when upregulated. We hypothesize that mRNA levels of ARE-containing mRNAs will be elevated in MNs and microglia expressing mutant HSPB1 compared to wild type HSPB1. To test this, we will directly measure ARE-containing mRNAs in MNs and microglia in mice. The development of these animals and the identification of the molecular function of HSPB1 that is important for MN survival will lead to new therapeutic targets and treatments for patients with HMN and has great potential to advance our understanding of and provide novel treatment strategies for all MNDs.